1. Field of the Invention
This invention resides in the field of gel electrophoresis, and addresses in particular the concerns and difficulties associated with attaching labels, markings and other indicia to slab gels for various purposes.
2. Description of the Prior Art
Gel electrophoresis is one of the most widely used analytical procedures in biotechnology for the separation of both proteins and nucleic acids from complex samples. Gel electrophoresis offers a sensitive, rapid means of separating, identifying and quantifying biologically relevant molecules. The configuration and size of the gel can vary, depending on the type of separation. Slab gels, for example, are particularly useful since they permit separations to be performed in either one or two dimensions, and the resulting bands or spots can be observed directly or visualized, recorded and analyzed by instrumentation. In one-dimensional slab gel separations, a large number of samples can be separated side-by-side in a single gel and analyzed simultaneously. In two-dimensional slab gel separations, thousands of proteins from a single sample can be resolved in a single gel typically by first using isoelectric focusing to separate the proteins into bands in one dimension and then using sodium dodecylsulfate polyacrylamide gel electrophoresis to further separate the proteins in each band in the second dimension. Tube gels or strip gels are typically used for the first dimension separation. Once separated, the molecules of interest can be detected and identified or quantified directly by comparison to standards, or interrogated further by diverse methods such as hybridization to tagged probes, immunochemical detection or analysis by mass spectrometry.
The reading of a gel can be performed either visually by the user or by automated detection systems. Certain indicia are often included in the reading procedure to assist in the identification of the sample components and to minimize the occurrence of errors. These indicia may consist of gel orientation guides, migration distance indicators, lane indicators, sample identifiers, the supplier's catalog numbers, and even the supplier's logo. Barcodes may be used to provide some of these indicia, while others may be simple letters, numbers or grid lines and similar markings. Indicia can also help avoid mistaking one gel for another when different gels have been simultaneously stained and washed, as is typically done prior to the reading of a gel to make the bands or spots readily detectable. Many of these concerns are present in tube gels as well, particularly those that serve as the first dimension in a two-dimensional separation. Tube gels would benefit not only from identifying indicia, but also from alignment markings that would alert the technician when a portion of the gel has been stretched or otherwise distorted.
Indicia can thus be useful in many ways, but to be effective, the indicia must not interfere with the electrophoretic separation or impose limitations on the processing and handling of the gel that occur after the separation is completed. These needs have presented a challenge to gel manufacturers and users.
A common means of applying indicia in the prior art is the embedding of a paper label in the gel. As the gel becomes enlarged during processing, however, paper labels often curl and tear, and even when they remain intact, paper labels tend to become stained, obscuring the indicia. A gel labeling method that does not involve the use of paper labels is disclosed in United States Patent Application Publication No. US 2003/0038030 A1, publication date Feb. 27, 2003, filing date Sep. 20, 2002. The method in this publication involves the use of a backing consisting of a solid transparent sheet on one side of the gel. The gel is chemically bonded to the backing to assure secure adherence, and the desired indicia are imprinted on the backing sheet. The gel is formed over the backing by polymerizing the gel from a monomer solution that is in direct contact with the backing while forming covalent bonds between the gel and functional groups on the backing. Other backing sheets are disclosed in U.S. Pat. No. 4,415,428, issued Nov. 15, 1988, on an application filed Jan. 27, 1982.
Backing sheets have also been used that adhere to the gel by means other than covalent bonds. Unfortunately, backing sheets that are not bonded to the gel are susceptible to detachment from the gel, particularly during the handling or processing of the gel after electrophoresis, which results in loss of the indicia. Regardless of how the backing sheet is made to adhere to the gel, the sheet itself presents problems during the staining of the gel. Staining is typically performed by immersing the gel in a staining solution and allowing the solution to penetrate the gel from all sides. When this is done with gels that have a backing sheet, penetration occurs only from the exposed side, and this requires more time for proper staining to occur. This results in low efficiency, sensitivity and reliability.
It would seem that some of these problems could be eliminated by using only a partial backing sheet or a label of smaller dimensions than the gel, leaving the remainder of the gel exposed on both sides. This would not be suitable for all types of indicia, but for those for which a small label would suffice, further problems would arise. When a gel to which a small label is attached is removed from a gel cassette for staining and analysis, for example, the chances that the gel will break are particularly high. One reason is that the gel in the area adhering to the label is less flexible and less able to stretch than areas of the gel that are not constricted by the label, and this difference makes the gel particularly vulnerable to breakage at locations close to the edges of the label. This vulnerability is made even greater when the gel is immersed in solutions for applying stains and for removing excess stain, since these solutions can cause a gel to enlarge or shrink by as much as 30%. Gels with small labels are also susceptible to the formation of extensive cracks emanating from the label when the gels are dried for long-term storage.